The present invention relates to a feed device for a sewing machine, for directly driving a feed dog with a stepping motor, capable of a function to prevent the asynchronism of the stepping motor.
The following three types of feed devices for sewing machines have been known.
A first feed device has a feed shaft driven by the main motor of the sewing machine, for driving the needle for vertical reciprocation. The feed per stroke and feed direction of the feed device are controlled by mechanically regulating a feed regulating device included in an interlocking mechanism for transmitting power from the upper shaft to the feed shaft, to regulate the amplitude of swing motion and phase of the interlocking mechanism relative to the rotation of the upper shaft.
A second feed device has a feed shaft driven by the main motor of the sewing machine. The feed per stroke and feed direction of the feed device are controlled electrically by driving a feed regulating device included in an interlocking mechanism interlocking the upper shaft and the feed shaft by a stepping motor or the like.
A third feed device has a feed shaft driven by an individual stepping motor provided in addition to the main motor. The rotation of the feed shaft, feed per stroke and feed direction are controlled by the stepping motor or the like.
The first and second feed devices have an advantage that a large feed force is available, because the feed shaft is driven for rotation by the main motor having a large torque. However, since the rotation of the upper shaft is converted into the rocking motion of a rod through an interlocking mechanism including a cam and a bifurcate link to rotate the feed shaft, the phase of the feed shaft relative to that of upper shaft is defined uniquely by the shape of the cam, and it is impossible to rotate the feed shaft through a large angle of rotation by the rotation of the upper shaft through a small angle of rotation due to restrictions on the mechanism, such as the limit of the pressure angle of the cam. That is, there is a tendency that the phase of the upper shaft at the start of the horizontal movement of the feed dog is advanced from an ideal phase, and the phase of the upper shaft at the end of the horizontal movement of the feed dog is retarded from an ideal phase. Consequently, it is possible that the feed motion is started before a stitch has firmly been tightened resulting in unsatisfactory tightening of the needle thread. Furthermore, in sewing a thick fabric, it is possible that the needle is thrusted into the fabric before the completion of the feed operation resulting in the breakage of the needle. Still further, since the constitution of the transmission mechanism interlocking the upper shaft and the feed device is complicated, the stitch pattern is deformed or irregular stitches are formed due to cumulative error in the transmission mechanism and enhancement of the cumulative error by friction betweeen the components of the transmission mechanism.
U.S. Pat. No. 4,286,532 discloses an improved device eliminated of the disadvantages of the foregoing conventional feed devices. This device drives the feed shaft directly for rotation by an individual stepping motor provided in addition to the main motor for driving the upper shaft. This device is designed to carry out the optional control of the feed operation by directly driving the stepping motor in an open loop control mode in synchronism with the upper shaft on the basis of the detected phase of the upper shaft. The optional control of the feed shaft enables the formation of complex patterns. In the open loop control mode, the stepping motor is controlled by commanding the relative angle of rotation by the number of stepping movement of the stepping motor. Accordingly, in operating the sewing machine, first the origin of the feed shaft is set, and then the number of command pulses is controlled on an assumption that the output shaft of the stepping motor rotates in correct response to the command pulses.
However, the open loop control of the stepping motor has a disadvantage that the step-out of the stepping motor occurs readily when an excessive load is applied to the stepping motor. Since the feed shaft rotates within a predetermined angular range, once the step-out of the stepping motor occurs, the absolute angle of the feed shaft becomes unknown, and the feed start angle and the feed stop angle are deviated from ideal angles in the direction of action of the load. Consequently, even after the excessive load has been removed, the control unit continues a predetermined control operation on an assumption that the step-out of the stepping motor never occurs. Therefore, when the feed shaft is required to be rotated through an angle corresponding to the number of command pulses in the normal or reverse direction, the angle of rotation of the feed shaft is restricted by the upper or lower limit of a predetermined angular range, which causes the stepping motor to step out. On the other hand, when the feed shaft restrained from rotation in one direction at the limit angle of the angular range due to the step-out of the stepping motor needs to be rotated in the opposite direction for return movement, in some cases, the direction of action of the torque of the stepping motor is reverse to the desired direction of rotation of the feed shaft depending on the mode of magnetization of the stepping motor. Consequently, the feed shaft is unable to rotate in synchronism with the command pulse signal given by the control unit. Thus, in such a state, the stepping motor is unable to respond to several initial pulses of the command pulse signal commanding the rotation of the output shaft of the stepping motor in the movable direction and, some times, the step-out of the stepping motor occurs. Accordingly, even after the excessive load has been removed, the step-out of the stepping motor occurs intermittently in part of the feed cycle until the origin of the feed shaft is determined, and hence the sewing machine is unable to from stitches corresponding to command signals.
In order to prevent the step-out of the stepping motor, an interlocking mechanism interlocking the feed dog through a spring with the output shaft of the stepping motor has been contrived to obviate the direct action of an excessive load working on the feed dog on the stepping motor. When this interlocking mechanism is employed, the torque of the stepping motor must be greater than the maximum working resilience of the spring. That is, the spring needs to have a small spring constant or the stepping motor needs to have a large torque capacity. However, when a spring having a small spring constant is employed, the synchronous operation of the feed dog with the rotation of the stepping motor is readily broken by a small load, and thereby a faulty pattern is stitched; and the torque capacity of the stepping motor is not utilized effectively. Employment of a large stepping motor having a large torque capacity is disadvantageous in respect of weight and space.